It is known that when some anisotropic materials with an appropriate molecular conformation are excited in a magnetic resonance experiment, the primary relaxation mechanism is dipolar coupling. It is further known that theory predicts that there is an optimum angle θ of primary field orientation of the main field relative to the object in order to obtain the strongest signal corresponding to the longest T2 relaxation rate (as the term 3 Cos2 θ−1 in the theoretical relationship goes to zero). The value of θ at which this occurs (around 54.7°) is sometimes referred to as the ‘magic angle’. Tendons are one such example in a medical application. However, similar considerations apply with non-medical anisotropic subjects.
The basic physics of this phenomenon is extensively described in Abragam A. The Principles of Nuclear Magnetism. Oxford University Press, Oxford, UK. 1961; Chapter IV. It was described in the context of MRI, using excised tissue, in Fullerton G D, Cameron I L, Ord V A, Orientation of tendons in the magnetic field and its effect on T2 relaxation times. Radiology 1985; 155(2):433-435. An account of the effect in vivo was reported in Oatridge A, Herlihy A H, Thomas R W et al. Magnetic resonance: magic angle imaging of the Achilles tendon, The Lancet 2001; 358(9293):1610-1611.
A conventional MRI machine has an annular magnet bore or employs a C-shaped yoke, such as shown in GB-A-2 282 451, so that the subject is positioned between the pole pieces at the end of the ‘C’. In such conventional situations, the space for positioning the patient or part of the patient is typically no more than about 60 cm across. It can therefore be appreciated that when it is necessary to orient the part of the patient at an optimum angle, the room for manoeuvre can be very limited, e.g. when taking image slices of joints such as knees or shoulders. At the very least, this can cause severe patient discomfort and at worst, the optimum orientation can be impossible to achieve.
In the case of the C-shaped yoke, the situation would be considerably improved if the whole magnet assembly could be pivoted relative to the patient. However, this is not really possible in view of the size and more particularly, the weight of the apparatus.
A novel form of electromagnet assembly has now been devised which is sufficiently light to enable it to be moved, and in particular oriented relative to the subject or patient so that the optimum angle is much more easily achieved. In this arrangement, each of a pair of coplanar coils is wound in opposite sense to the other and coils in a second coplanar pair, parallel to the first pair, are wound likewise but in mirror sense thereto.
Use of planar gradient coils in an MRI machine is known from U.S. Pat. No. 5,867,027 and SU-A-1804616. Pairs of coplanar r.f. coils are disclosed in U.S. Pat. No. 6,975,115 and JP-A-7303621.
GB-A-2 355 799 discloses an electromagnet assembly having a pair of main windings whose currents in the upper and lower planes are in opposite senses to achieve a main field parallel to the plane of the coils. Each plane of the main field coils is a pair with mirror symmetry about the X axis giving automatic symmetry about the Z=0 plane. With the present invention, the main field generating elements are parallel and in the same sense creating an efficient generation of net field. In contrast, the arrangement described in GB-A-2 355 799 has neighbouring elements in opposite senses and so tend to cancel, making it very inefficient. In a particularly preferred class of embodiments of the present invention there is an additional plane of symmetry about the Z=0 plane (in the axis labelling notation employed herein, equivalent of plane X=0 in OMT's notation) which is extremely valuable in creating an intrisically homogeneous magnet. All field-creating elements nearest the isocentre in all 8 octants of 3d space generate field in the same sense.